This invention relates generally to scanning electron microscopes, and more particularly, to a method and apparatus for automatic electron beam alignment and astigmatism correction in scanning electron microscopes.
An electron microscope is capable of resolution and magnification far beyond that of an optical microscope. The resolution limit of a microscope, whether optical or electron, depends upon the wavelength of the imaging wave. The shorter the wavelength, the smaller the limit of resolution of the microscope. For example, objects that are smaller than the wavelength of visible light, roughly 500 nm, are bypassed by light waves and cannot be clearly resolved by an optical microscope.
Electron microscopes, on the other hand, use electron waves instead of light waves as imaging waves. Electron waves occur at wavelengths much shorter than those of visible light. For example, a typical wavelength of an electron wave is about 0.005 nm. The electron microscope's resolution is limited by the size of the electron beam striking the sample. The resolution limit of an electron microscope can be hundreds of times smaller than the resolution limit of an optical microscope. Because an electron microscope can resolve much smaller objects than can an optical microscope, an electron microscope is capable of magnification hundreds of times beyond that of an optical microscope.
Two kinds of electron microscopes include a transmission electron microscope ("TEM") and a scanning electron microscope ("SEM"). The transmission electron microscope forms an image of a specimen by passing electrons through the specimen. A scanning electron microscope generates a small electron beam, focuses the beam with a condenser lens and an objective lens, and forms an image of a specimen by scanning the specimen and detecting secondary electrons emitted from the surface of the specimen.
In a scanning electron microscope, it is important to correctly align the electron beam axis to the axis of the lenses. A misalignment between the electron beam axis and the axis of the lenses results in distortion of the beam, increased final spot size of the beam, and decreased resolution of the SEM. Conventionally, alignment of the objective lens is performed manually by an operator. According to known beam alignment techniques, an operator images a feature of a specimen using a scanning electron microscope. Typically, a sputtered gold-on-carbon standard is used as the specimen. The sputtered gold gives a high yield of secondary electrons, while the carbon does not. Secondary electrons are electrons emitted by the specimen in response to bombardment by the primary imaging system. They typically have energies that are 10%-25% of the incident beam. The operator simultaneously (1) manually adjusts the focus of the objective lens between long and short extremes of the focal range by adjusting current or voltage supplied to the electromagnetic or electrostatic objective lens, respectively; (2) manually adjusts beam alignment by adjusting current or voltage supplied to electromagnetic or electrostatic alignment coils, respectively; and (3) looks for translation of the image of the specimen. In some prior art alignment methods, the objective lens focus is automatically varied between extremes of the focal range (commonly referred to as "wobbling"). When the beam is properly aligned, the operator can perceive no translational motion of the imaged feature. With proper beam alignment, an operator can perceive only a "breathing effect," or magnification and shrinking of the image of the specimen, due to variations in objective lens focus.
One shortcoming of this conventional beam alignment technique is that the manual alignment is highly dependent on operator judgment. For example, perceptions of translational motion may vary between operators. This approach therefore introduces operator inaccuracies and interoperator variations.
In a scanning electron microscope, it is also important to correct any astigmatism in the electron beam. An astigmatism in the electron beam is a focus defect in which the electron beam is not radially uniform. This causes a decrease in the resolution of the image. Connecting an electron beam astigmatism requires identifying an axis of electron beam distortion, and then adjusting radial uniformity in the axis of electron beam distortion to provide a circular electron beam. This adjustment is made via an astigmatism coil. Conventionally, electron beam astigmatism correction is performed manually by an operator. According to known astigmatism correction techniques, an operator images a feature of a specimen using a scanning electron microscope. The specimen is commonly a gold-on-carbon standard, which provides a limited amount of contrast along orthogonal axes. The operator subjectively evaluates sharpness of the imaged feature, and introduces corrections to radial uniformity of the electron beam until a satisfactory image is obtained. Here, a "satisfactory image" means a "sharp" image that is not "blurred," as subjectively determined by the operator.
One shortcoming of this conventional beam astigmatism correction technique is that it also is highly dependent on operator judgment. For example, determinations of what constitutes a satisfactory image may vary between operators. This approach therefore introduces operator inaccuracies and interoperator variations.
Accordingly, there is a need for a more accurate, more consistent, and automated method for electron beam alignment and astigmatism correction in scanning electron microscopes.